a. Field of Invention
The present invention relates to a process for application of layers to sputter targets or X-ray anodes, which layers contain only small amounts of gaseous impurities, such as oxygen.
a. Description of the Prior Art
The application of refractory metal layers to surfaces displays numerous problems.
In the familiar processes, the metal is usually melted completely or partly, as a result of which the metals readily oxidize or take up other gaseous impurities. Conventional processes, such as deposit welding and plasma spraying, must therefore be carried out under an inert gas or in vacuo.
In this case, a high outlay on apparatus is necessary, the size of the structural components is limited and the content of gaseous impurities here is still not satisfactory.
The introduction of large amounts of heat transmitted into the object to be coated leads to a very high potential for distortion and ensures that these processes cannot be employed in the case of complex structural components, which often also comprise components which melt at low temperatures. Such structural components are, in particular, so-called sputtering targets, that is to say sources of metal which are employed in cathode sputtering of metal. Complex structural components must therefore be dismantled before processing, which as a rule means that processing is uneconomical in practice, and merely recycling of materials (scrapping) of the structural components is carried out.
In vacuum plasma spraying, impurities of tungsten and copper which originate from the electrodes used are moreover introduced into the layer, which as a rule is undesirable. For example, if layers of tantalum or niobium are used for corrosion protection, these impurities reduce the protective action of the coating by the formation of so-called micro-galvanic cells. In the case of sputter targets, this contamination can lead to components becoming unusable.
These processes moreover are melt metallurgy processes, which always involve their inherent disadvantages, such as, for example, unidirectional particle growth. This occurs in particular in laser processes, wherein a suitable powder is applied to the surface and is melted by a laser beam. A further problem lies in the porosity, which can be observed in particular if a metal powder is first applied and this is then melted with a heat source. It has indeed been attempted in WO 02/064287 to solve these problems by merely superficially melting and sintering the powder particles by a beam of energy, such as e.g. laser beams. However, the results are not always satisfactory, a high outlay on apparatus is necessary and the problems associated with an indeed reduced but nevertheless high introduction of heat into a complex structural component remain.
WO-A-03/106,051 discloses a method and an apparatus for low pressure cold spraying. In this process a coating of powder particles is sprayed in a gas substantially at ambient temperatures onto a workpiece. The process is conducted in a low ambient pressure environment which is less than atmospheric pressure to accelerate the sprayed powder particles. With this process a coating of a powder is formed on a workpiece.
EP-A-1,382,720 discloses another method and apparatus for low pressure cold spraying. In this process the target to be coated and the cold spray gun are located within a vacuum chamber at pressures below 80 kPa. With this process a workpiece is coated with a powder.